1. Field of the Invention
The present invention relates to thermosetting powder coating compositions, useful as film-forming materials. More particularly, the present invention relates to thermosetting powder coating compositions containing bisphenoxy-propanol as the melt viscosity modifier.
2. Description of Related Art
Thermosetting compositions, including epoxy, acrylic and polyester resins, and various crosslinking agents, are known to be useful for coatings. The compositions should be in powder form, with an average particle size typically in the range of from 40 to 120 micrometers and should be physically and chemically stable during storage at ambient temperature for a prolonged period of time (usually up to 12 months). They are typically electrostatically applied to objects and heated to temperatures in the range from 120.degree. to 240.degree. C., at which point they fuse and undergo chemical reactions forming a durable, uniform, crosslinked, and insoluble film.
U.S. Pat. No. 4,065,438 describes thermosetting powder coating compositions comprising carboxyl functional polyesters and bisphenol A-type epoxy resins. These compositions have been found to result in coatings which exhibit excellent mechanical properties, such as flexibility and impact resistance.
U.S. Pat. No. 4,147,737 is directed to carboxyl functional polyester powder coatings crosslinked with triglycidyl isocyanurate. Such powder coating compositions have very good mechanical properties and relatively good outdoor durability. Alternatively, carboxlic polyester can be cured with hydroxyalkyl amides.
U.S. Pat. No. 3,931,117 discloses a process for the preparation of hydroxy functional polyester resins which, in combination with blocked isocyanates or melamine resins, are suitable for producing thermosetting powder coatings.
U.S. Pat. No. 4,181,642 describes glycidyl methacrylate copolymers as the basic polymers for the preparation of powder coating compositions. Such compositions can be used as ultraviolet light resistant coatings.
U.S. Pat. No. 4,286,021 discloses the use of low molecular weight acrylic copolymers based on isobornyl methacrylate as a melt viscosity modifier in powder coating systems.
Notwithstanding the disclosures in the above references, a continual need in the area of powder coating is to form a cured film with substantially no visible defects. Such defects are typically in the form of a poor flow, cratering, orange-peel effect and pin-holes. The flow can be improved and cratering and orange-peel effect eliminated by the use of flow control agents. However, the elimination of pin-holes requires special additives able to lower the melt viscosity of the powder coating composition during the curing process.
Acrylic additives described in U.S. Pat. No. 4,286,021 are capable of lowering the melt viscosity of the system, but do not eliminate or reduce the formation of pin-holes. Benzoin is also used as an additive in powder coating formulations, but it has a strong tendency to discoloration (yellowing) under the influence of heat.